1. Field of the Invention
The present invention is directed to circuit board testers, and more particularly to a spring-loaded, removable test fixture for circuit board testers.
2. Description of the Related Art
Circuit board testers are used for testing a variety of circuit boards or similar devices to assure that the circuit boards operate as intended. In at least one type of circuit board tester, such as Agilent Model No. 3070, Series 3, a separate device, referred to as a fixture, is used to position the circuit board such that a plurality of electrically conductive probes (which are part of, or coupled to, the tester) contact predetermined components or positions of the circuit board. The particular components or positions that are contacted by the test or probes depend on the tests that are desired. When the probes are in contact with the desired locations on the circuit board, electrical signals with predetermined parameters (e.g., predetermined magnitudes or patterns of current, voltage frequency, phase and the like) are applied by the tester, typically under control of a computer, to certain of the probes. Some or all of the probes are used to measure the performance or response of the circuit board (i.e., to measure electrical parameters at some or all of the probes contacting the circuit board). In this way, it is possible to rapidly perform a number of tests or measurements characterizing the performance of the circuit board while simulating the conditions the circuit board would have, or could have, during actual use. Although it is possible to use these types of tests (and testing devices) for a variety of possible purposes (such as “spot checking” selected circuit boards at a production facility, testing circuit boards which may be malfunctioning, testing prototype circuit boards as part of a design program and the like), in at least some applications, circuit board testing is used to provide quality assurance on all or substantially all products of a given type or class which are produced by a company. Even with the relatively rapid test procedures which can be achieved by in circuit testing, it is not unusual for desired testing of each circuit board to require on the order of 30 seconds to 90 seconds or more.
Generally, the tests are performed in the following sequence. First, a unit under test (UUT) is removably attached to a mechanical fixture on one side of the UUT, say, the top side. The bottom side of the UUT is left exposed, and contains the majority of the electrical circuitry to be tested. If the UUT contains a heat sink, the heat sink is generally located on the top side of the UUT, and there is generally clearance in the mechanical fixture to accommodate the heat sink, if required.
Once the UUT is rigidly attached to the fixture, the fixture is rigidly attached to the upper lid of the tester, often called a vacuum box. Before any tests are performed, the UUT is proximate, but not in contact with a series of electrical probes that are attached to a probe plate. The probe plate and probes are generally located beneath the UUT, on the side opposite the heat sink and opposite the vacuum box. The UUT and the electrical probes are then brought into contact by evacuating the region between the vacuum box and the probe plate, forcing the vacuum box and the probe plate toward each other due to atmospheric pressure. While the UUT and the electrical probes are in contact, the tester applies and measures a series of voltages and currents through the electrical probes at various locations on the UUT, in order to test the UUT's electrical characteristics and ensure proper performance. When the tests are completed, the vacuum is released, the UUT and the probes become spaced apart, and the UUT may be removed from the fixture.
The UUT may or may not contain its own heat sink, and the specific tests may or may not require that such a heat sink be removed during the test. Therefore, the circuit board testers are designed for (generally) one of two categories: (1) UUT with heat sink, and (2) UUT without heat sink.
For category (1), in which the UUT is tested with its heat sink attached, the tester should deliberately contain a mechanism for heat dissipation. (Recall that the testers generally operate in a near vacuum, which inhibits the effects of convection.) Typically, the tester has an interface that is brought into contact with the heat sink when the UUT is rigidly attached to the fixture. At the interface, the heat sink of the UUT is brought into thermal contact with a thermal conductor in the vacuum box lid, which generally directs heat through a port in the lid to a heat dissipater located outside the tester, nominally at room temperature and pressure. The tester interface is permanently attached to the vacuum box lid, and is typically designed to accommodate only one dimension of heat sink.
If the height of the heat sink changes in a future revision of the UUT, the thermal interface should be retooled. Furthermore, part-to-part variations in the heat sink height may cause problems during testing. If the heat sink on the UUT is too tall, it may lead to mechanical damage to the UUT or to the tester. If the heat sink on the UUT is too short, it may lead to poor (or no) thermal contact and, subsequently, poor test results. Accordingly, it would be beneficial to have a configuration in which the thermal interface is spring-loaded, in order to accommodate part-to-part variations in UUT heat sink height. It would also be beneficial to have a removable configuration, in which the thermal interface may be readily swapped out, in order to accommodate any design changes for the heat sink height.
For category (2), in which the heat sink is absent from the UUT, the tester generally does not provide for heat dissipation. Instead, the tester may perform additional electrical tests on the side of the UUT opposite the probe plate. Generally, these additional tests are not as rigorous as those performed by the electrical probes, and the function of many of the additional electrical tests is to check the performance of some of the discrete components mounted on the circuit board, such as a capacitance measurement for discretely mounted capacitors, and so on. There exist several off-the-shelf modules that assist in these measurements, such as the TestJet Assembly, manufactured by Hewlett-Packard. The module is held rigidly against the top side of the UUT (opposite the probe plate) by a series of receptacles. The receptacles affix the module to the vacuum box, and are generally spring-loaded, but not adjustable or removable. The module is electrically connected via the receptacles through a port in the vacuum box to an electrical contact pin, which in turn is connected to controlling circuitry external to the vacuum box.
As with category (1), any height variations in the UUT may cause improper test results or damage to the tester. Accordingly, it would be beneficial to have a spring-loaded, removable interface at the vacuum box, both for the thermal interface of category (1) and for the electrical interface of category (2).